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Spotlight on Optics

January 2018

Spotlight Summary by Thomas Gerrits

Asavanant et al. demonstrate the deterministic generation of high-purity coherent state superpositions (CSS) in time derived from a continuous wave (CW) squeezing source and photon subtraction. The authors refer to the generated states as Schrödinger cat states. Schrödinger cat states are fundamental quantum states with no classical analogues. Schrödinger’s Gedanken experiment of 1935 described an actual cat held in a superposition of alive and dead states. We now use “Schrödinger cat” to refer to a quantum state that is a superposition of two highly distinguishable classical states. Such states could be useful for quantum information processing and metrology beyond the quantum limit. Generation of these states in travelling optical modes is especially desirable, because they can be used for quantum communication, computing, and interferometry.

The non-Gaussian operation of photon subtraction from a squeezed vacuum projects the squeezed vacuum into a squeezed single photon state, a close approximation to the desired optical Schrödinger cat state. High purities can be achieved with such CW sources. However, the state’s location in time could only be determined in post-processing until now. Asavanant et al. demonstrate CW-squeezing mode matching through optical and electrical filtering, and consequently observe a cat state in real-time without the need of post-processing. Thus, the cat state generated from the CW squeezer has a deterministic appearance in time, in principle allowing for feed-forward operation and interference on beam splitters, a necessary ingredient for continuous variable quantum computing.